1. Field of the Invention
The present invention relates generally to a surface acoustic wave filter and, more particularly, to a surface acoustic wave filter having a widened band and a lowered insertion loss, advantageous to application to a high-frequency circuit of a radio apparatus.
2. Description of the Related Arts
The high-frequency circuit of the radio apparatus typically uses a filter. Wide-band filter characteristics are required for an SAW (surface acoustic wave) device which is an indispensable key device for the filter.
A surface acoustic wave filter formed of such an SAW (surface acoustic wave) device includes a piezoelectric substrate on which are formed interdigital transducers (IDTS) having interdigital electrodes and reflectors having grating electrodes. It is commonly known that the widened band of the surface acoustic wave filter is achieved by increasing the thickness of the electrodes. However,increased electrode thickness may impair the insertion loss which is a key characteristic of the filter.
In this manner, the electrode thickness has hitherto been increased to achieve the widened band in spite of possible deterioration of the insertion loss and further of the passband arising from impedance mismatching.
It was therefore an essential problem to realize the wide band and low insertion loss of the surface acoustic wave filter.
In view of the above problems, an object of the present invention is attained on the assumption that use is made of an electrode thickness capable of widening the band and that use is made of optimum cutting orientation for the piezoelectric substrate.
It is another object of the present invention to provide a surface acoustic wave filter free from any loss arising from the impedance mismatching by finding out optimum values achieving a widened band and lowered loss for the electrode width of the interdigital transducer electrodes and reflector electrodes.
According to an aspect of the present invention, to achieve the above object there is provided a surface acoustic wave filter having interdigital electrodes formed on a piezoelectric substrate, the surface acoustic wave filter including an interdigital transducer having interdigital electrodes and reflectors arranged on both sides of the transducer, wherein a ratio h/xcex of thickness h of the interto a surface acoustic wave wavelength xcex lying within the range of 0.05xe2x89xa6h/xcexxe2x89xa60.15, and where in the piezoelectric substrate has an electrode width ratio wr/pr of a reflector electrode width wr to a reflector electrode pitch pr lying within the range of 0.5xe2x89xa6wr/prxe2x89xa60.6, and wherein the piezoelectric substrate has an electrode width ratio wi/pi of an electrode width wi of the interdigital transducer to an electrode pitch pi of the interdigital transducer lying within the range of 0.6xe2x89xa6wi/pixe2x89xa60.9.
Preferably, to achieve the above object, when the ratio wi/pi of the electrode width wi to the pitch pi of the interdigital electrodes of the transducer lies within the range of 0.62xe2x89xa6wi/pixe2x89xa60.9, the relationship between the electrode width wi of the interdigital electrodes and the electrode width wr of the reflectors is given as wi greater than wr and the relationship between the electrode pitch pi of the interdigital electrodes and the electrode pitch pr of the reflectors is given as pi less than pr.
Preferably,to attain the above object, when the distance between centers of adjoining electrode fingers of the interdigital electrodes and of the reflector electrodes is given as (xc2xd+HD1)xc3x97xcexi where xcexi is a wavelength in the interdigital electrodes, the coefficient HD1 is defined in the range of 0 greater than HD1xe2x89xa7xe2x88x920.04.
Preferably, to attain the above object, the piezoelectric substrate is formed by a rotational Y-plate of LiTaO3 obtained by rotating Y-axis through 40xc2x0 to 44xc2x0 around X-axis in the direction of Z-axis, and the interdigital electrodes of the transducer and electrodes of reflectors are formed of an electrode material made mainly of Al.